1. Field of Art
The present invention relates to a highly water absorbing composite sheet and in particular, a highly water absorbing composite sheet with a highly water absorbing solid resin held in a non-woven substrate sheet and to a method of manufacturing the same. The present invention also relates to absorbent products using such highly water absorbing composite sheet.
In the present invention every shape of solid substances including particles, pellets, film or non-woven fabric like shape can be used as the highly water absorbing resin. In this specification, the term xe2x80x9chighly water absorbing solid resinxe2x80x9d or xe2x80x9csolid SAPxe2x80x9d is used to mean highly water absorbing resins of every such shape.
2. Related Art
Highly water absorbing composite sheet comprising a non-woven substrate sheet and solid SAP held on the surface of the non-woven substrate sheet has been used as an absorbent component in such absorbent products such as baby diapers, adult diapers, sanitary napkins, blood absorbent and mother""s milk pads. Such highly water absorbing composite sheet is described in several patents such as U.S. Pat. No. 5,147,343.
Until now, in this type of highly water absorbing composite sheet, the fixing of solid SAP to a non-woven substrate sheet has been done by means of adhesiveness of a hot-melt adhesive. Or a method has been applied of coating the non-woven substrate sheet with a suspension of the solid SAP or a suspension of a mixture of the solid SAP and pulp. In the case of the solid SAP and pulp mixture system, the fixing of the solid SAP to the non-woven substrate sheet depends on the self-adhesiveness of the pulp.
In case a suspension containing the solid SAP is used, a suspension may sometimes be used where easy-to-thermally-melt binder fibers (for example, bicomponent fibers) are added. This suspension is applied on the non-woven substrate sheet and heated and then cooled down whereby the solid SAP and the pulp, if any, are fixed into the non-woven substrate sheet by means of the easy-to-thermally-melt binder fibers.
Another method of fixing the solid SAP into the non-woven substrate sheet is that easy-to-thermally-melt fibers or fiber web containing such easy-to-thermally-melt fibers are made to contain solid SAP and by means of heat treatment the fibers constituting such fiber web are fused together with the result that the solid SAP is fixed into the substrate.
In the conventional technology, however, in the case of the solid SAP and pulp mixture system in particular, it is difficult to increase the ratio of solid SAP/pulp (xe2x80x9cSAP ratioxe2x80x9d) to a great extent and thus the maximum ratio has been around 50% by weight. In the system where the solid SAP is fixed by means of the binder, the swelling capacity of the solid SAP and the binding capacity of the solid SAP by means of the binder work in antagonistic action with each other. That is to say, the higher the binding capacity of the solid SAP, the more the swelling of the solid SAP is impeded and inversely if the swelling is less impeded, the binding of the solid SAP becomes more difficult.
Therefore, a primary object of the present invention is to provide a structure where as the solid SAP swells the substrate at the same time swells; that is to say, a structure where while the solid SAP and the substrate are so loosely bonded with each other as for the solid SAP to maintain its degree of freedom, the swollen solid SAP is so contained by the substrate that the solid SAP does not go away from the substrate.
According to the present invention, a highly absorbent composite sheet is provided comprising a non-woven fabric substrate, solid SAP and a thermally fusible component, characterized in that:
said non-woven substrate has a bulky structure;
part of said solid SAP is contained inside said bulky structure and the rest is exposed on the surface of said non-woven substrate;
said thermally fusible component is a hot-melt adhesive;
said hot-melt adhesive forms a fibrous network; and
said fibrous network covers said solid SAP in contact with said solid SAP whereby said solid SAP is held in position.
The preferred range of the coated amount of said hot-melt adhesive is 0.2 to 10 g/m2. 
The hot-melt adhesive is preferably of non-tacking type; for example, an adhesive comprising as a main component a copolymer of ethylene and vinyl acetate is most preferable. The content of vinyl acetate in the ethylene /vinyl acetate copolymer is preferably 20 to 40% by weight and its thermal fluid coefficient is preferably 50 to 150 g/10 min.
According to the present invention, a method for manufacturing a highly absorbent composite sheet is provided comprising the steps of:
forming a bulky structure by raising a non-woven substrate;
applying slurry containing solid SAP to the raised surface of said non-woven substrate, then removing remaining liquid and drying whereby a part of solid SAP is contained in said bulky structure and the rest of solid SAP is exposed on the surface of said non-woven substrate; and
making a hot-melt adhesive fibrous by means of a curtain spray apparatus, then blowing the adhesive in the form of a curtain and forming a fibrous network on said non-woven substrate and said solid SAP.
Raising can be made in a variety of manners. A preferable way of raising is to make one surface of a non-woven substrate in contact with a heated roll and then to make it in contact with a chilled roll after it is removed from the heated roll.
All solid SAP is preferably contained by the bulky structure of said non-woven substrate, i.e. in voids between and among the fibers constituting the non-woven substrate. Although, depending on the amount of solid SAP added and on the bulkiness of a carded web used, part of the solid SAP may be exposed on the non-woven substrate, it does not get in the way of achieving the object of the present invention.
Such exposed solid SAP, however, may turn into dust and particles as abraded or bent when the non-woven substrate containing the solid SAP is slit or it is incorporated into a product although such solid SAP appears to be stable as it is left stationary even if it is in dry condition. And such exposed solid SAP may easily be separated from the composite when it is swollen in wet condition. For stabilizing the solid SAP in such dry and wet conditions only by virtue of the containing capability of the non-woven substrate, preferably, the non-woven substrate needs to be made more bulky and the solid SAP to be contained by the substrate needs to be less. In other words, generally speaking, the non-woven substrate alone can hardly contain 70% or more of solid SAP used and the added amount of solid SAP can hardly be more than 300 g/m2.
By utilizing a fibrous network based on the use of a hot-melt adhesive according to the present invention, such non-woven substrate as has not been so far used for being very often peeled off, for example, a spunbond (generally called xe2x80x9cSBxe2x80x9d) or a composite of a spunbond/meltblown/spunbond (generally called xe2x80x9cSMSxe2x80x9d) is made usable as an absorbent substrate.
According to the another aspect of the present invention, there is provided a highly absorbent composite comprising a composite absorbent (M) which comprises a non-woven substrate, a SAP layer and a hot-melt adhesive layer forming a fibrous network covering said SAP layer, and a sheet material (N) disposed on said adhesive layer, said composite absorbent (M) and said sheet material (N) being bonded together by said hot-melt adhesive layer by an adhesive property thereof to form a composite structure (M/N).
In place of the sheet material (N), it is possible to use another composite absorbents (Mxe2x80x2) having a same construction of the above composite absorbents. The composite absorbents (M) is laid on the other composite absorbent (Mxe2x80x2) in such manner that the hot-melt adhesive layers contact to each other and being bonded together by an adhesive property thereof to form a composite structure (M/Mxe2x80x2).
Alternatively, an additional sheet material (N) may be interposed between said composite absorbents (M) and (Mxe2x80x2) and bonded thereto by an adhesive property of the hot-melt layers of the composite absorbents (M) and (Mxe2x80x2) to form a composite structure (M/N/Mxe2x80x2).
In another mode of the present invention, is provided a highly absorbent sheet in which solid SAP is generally distributed in layers almost all over one surface of a non-woven substrate with part of it in the voids of the non-woven fabric and another part of it exposed off the surface wherein the surface of said exposed solid SAP layer is covered by a dual fibrous network structure consisting of a first fibrous network of fine mesh comprising a hot-melt adhesive and a second fibrous network disposed on said first network and of coarser mesh compared with said first network comprising a hot-melt adhesive layer so that the highly absorbent sheet is much less apt to peel off than conventional products.
In the highly absorbent sheet thus covered by the dual fibrous network, the solid SAP needs not necessarily be bonded with each other by the fine cellulose fibers.
In the above-mentioned configuration, the first fibrous network is of fine mesh and the second fibrous network disposed on the first fibrous network is of coarse mesh, but the relation between the two may be reversed, that is to say, the first fibrous network may be of coarse mesh and the second fibrous network disposed on the first fibrous network may be of fine mesh.
Preferably, the fibers of a hot-melt layer of fine mesh are finer than those of a hot-melt layer of coarse mesh.
According to the present invention, there is provided a method for manufacturing a highly absorbent composite sheet by treating the surface of the highly absorbent composite sheet in which solid SAP is distributed in layers on one surface of a non-woven substrate comprising a combination of:
a first stage hot-melt processing step in which a hot-melt processing is conducted by means of a hot-melt feeding apparatus (A) which forms a first fibrous network of fine mesh consisting of hot-melt adhesive on the surface where said solid SAP is distributed; and
a second stage hot-melt processing step in which a hot-melt processing is conducted by means of a hot-melt feeding apparatus (B) which forms a second fibrous network consisting of hot-melt adhesive of coarser mesh than said first fibrous network.
In a preferred aspect of the present invention said first stage hot-melt processing step is carried out with the hot-melt adhesive coated in an amount of 0.3 g/m2 to 2 g/m2 so that a first fibrous network of fine mesh is formed and said second stage hot-melt processing step is conducted with the hot-melt adhesive coated in an amount of 1 g/m2 to 10 g/m2 so that a second fibrous network is formed of coarser mesh than in the case of said first stage hot-melt processing step.
More preferably, said first stage hot-melt processing step is carried out with the hot-melt adhesive coated in an amount of 1 g/m2 to 10 g/m2 and said second stage hot-melt processing step is conducted with the hot-melt adhesive coated in an amount of 0.3 g/m2 to 2 g/m2 so that a second fibrous network is formed of finer mesh than in the case of said first stage hot-melt processing step.
A preferred form of said hot-melt feeding apparatus according to the present invention is two units of curtain spray type hot-melt feeding apparatus which is capable of forming a network of relatively fine mesh as combined in series with respect to the moving direction of said non-woven substrate.
By using a curtain spray type hot-melt feeding apparatus as said first stage hot-melt feeding apparatus a first fibrous network of fine mesh can be formed and by using a spiral coating type hot-melt feeding apparatus as said second stage hot-melt feeding apparatus a second fibrous network of coarser mesh than said first hot-melt layer can be formed.
Alternately, by using a spiral coating type hot-melt feeding apparatus as said first stage hot-melt feeding apparatus a first fibrous network of coarser mesh is formed and by using a curtain spray type hot-melt feeding apparatus as said second stage hot-melt feeding apparatus a second hot-melt layer of finer mesh than said first fibrous network is formed.
Furthermore, by using a curtain spray type hot-melt feeding apparatus as said first stage hot-melt feeding apparatus a first fibrous network of fine mesh can be formed and by using a line coating type hot-melt feeding apparatus as said second stage hot-melt feeding apparatus a second fibrous network of coarser mesh than said first fibrous network can be formed.
Alternately, it may sometimes be preferable that by using a line coating type hot-melt feeding apparatus as said first stage hot-melt feeding apparatus a first fibrous network of coarse mesh is formed and that by using a curtain spray type hot-melt feeding apparatus as said second stage hot-melt feeding apparatus a second fibrous network of finer mesh than said first fibrous network is formed.
A highly absorbent composite sheet according to the present invention is effectively used in absorbent products provided with a topsheet having liquid perviousness, an absorbent having liquid absorbency and liquid retention and a backsheet having liquid imperviousness as an absorbent having said liquid absorbency and liquid retention.